1. Technical Field
The present invention relates to an interleaving apparatus or interleaver for a Code Division Multiple Access (CDMA) mobile telecommunication system and, more particularly, to a base-band interleaver for forward traffic and paging channels preferably applicable to an IS-95 CDMA specification of telecommunication system.
2. Related Art
Common air interface (IS-95) standard is a mobile telecommunication standard which uses Code Division Multiple Access (CDMA) for signal transmission over communication channels. CDMA is a form of modulation technique for spread-spectrum multiple-access digital communications. In CDMA, a wide-band frequency channel is shared between several overlapping signals, each characterized by a specific pseudo random binary sequence that spreads the initial spectrum of the data to be transmitted.
In a mobile telecommunication environment, signal strength varies with location and movement of the mobile transmitter/receiver. Signal strength can significantly affect error rates which in turn effect the quality of communication. Due to varying signal strength, mobile telecommunication systems are susceptible to burst errors. Burst errors are groupings of errors that occur in adjacent bits of a data block as compared to errors that are dispersed over an entire block of data. A solution to varying signal strength and burst errors is to utilize an error correction scheme based on encoding and interleaving.
Generally, interleaving is used in conjunction with encoding (e.g., error-correcting codes) in order to lower the error rates of communication channels that are susceptible to burst errors. Interleaving is a technique in which encoded digital data is reordered before transmission in such a manner that any two successive digital data bits in the original data stream are separated by a predetermined distance in the transmitted data stream. Deinterleaving is the reverse of interleaving where data bits are reordered back to their original positions. According to the IS-95 standard, data is encoded and interleaved prior to transmission and decoded and deinterleaved after reception. Such deinterleaving disperses burst errors, which can occur during transmission throughout the data block. This dispersal of bit errors enables the decode to treat the burst errors as if they weie random errors, and maximize the capability of the decoder to correct the errors.
One class of interleavers is known as periodic interleavers, for which the interleaving permutation is a periodic function of time. Examples are block interleavers, which accept data in blocks and perform identical permutations over each data block, and convolutional interleavers have no fixed block structure, but perform a periodic permutation over a semi-infinite sequence of coded symbols. A block interleaver typically takes the coded symbols and writes them by columns into a matrix with rows and columns. The permutation consists of reading these symbols out of the matrix by rows prior to transmission. The deinterleaver then performs the inverse operation. Symbols are written into the deinterleaver by rows and read out by columns. In contrast to block interleavers, convolutional interleavers requires the coded symbols to shift sequentially into a bank of registers with increasing length. With each new code symbol, a commutator switches to a new register and the new code symbol is shifted in while the oldest code symbol in that register is shifted out to the communication channel. The deinterleaver has a similar structure and performs the inverse operation. Exemplars of contemporary block and convolutional interleavers are disclosed in U.S. Pat. No. 4,394,642 for Apparatus For Interleaving And De-Interleaving Data issued to Currie et al., U.S. Pat. No. 4,559,625 for Interleavers For Digital Communications issued to Berlekamp et al., U.S. Pat. No. 4,742,517 for Interleaving Circuit issued to Takagi et al., U.S. Pat. No. 5,042,033 for RAM-hnplemented Convolutional Interleaver issued to Costa, U.S. Pat. No. 5,056,112 for Interleaving In Coded Modulation for Mobile Radio issued to Wei, U.S. Pat. No. 5,136,588 for Interleaving Method And Apparatus issued to Ishijima, U.S. Pat. No. 5,241,563 for Method And Apparatus For Communicating Interleaved Data issued to Paik et al., U.S. Pat. No. 5,276,827 for Data Buffer For The Duration Of Cyclicaly Recurrent Buffer Periods issued to Delaruelle et al., U.S. Pat. No. 5,535,220 for Forward Error Correcting Transmitter And Receiver issued to Kanno et al., U.S. Pat. No. 5,537,420 for Convolutional Interleaver With Reduced Memory Requirements And Address Generator Therefor issued to Huang, U.S. Pat. No. 5,636,224 for Method And Apparatus For Interleave/Deinterleave Addressing In Data Communication Circuits issued to Voith et al., and U.S. Pat. No. 5,659,580 for Data Interleaver For Use With Mobile Communication Systems And Having A Contiguous Counter And An Address Twister issued to Partyka.
Many contemporary interleavers utilize a memory look-up table to determine the proper interleaving address. While the use of look-up table minimizes the processing capability required to generate interleaving addresses. However, the look-up table requires a significant amount of memory and memory space for implementation. Moreover, most contemporary interleavers for forward traffic channel and paging channel in a base-band defined in accordance with the IS-95 CDMA standard require a general-purpose memory of 512 bytes for the look-up table. However, I have noted that the look-up table usually requires only 384 bytes of the 512-byte memory chip, the remaining 128 bytes of memory may become wasteful.